In most non-centrosymmetrical crystalline bodies, the refraction indices, which generally depend on the propagation and polarization direction of the incident beam, vary linearly with an electric field applied externally. This is what is known as the Pockels effect, recognized since the end of the last century. This index variation may be advantageously used to change the phase of the luminous beam, the latter then being able to be measured by an interferometric system, this being the case in electro-optic modulators.
From this phase variation, in principle, it is possible to revert to the value of the electric field. By using a nonconducting crystal and without having any metallic piece in the sensor, it is thus possible to measure the electric field without disturbing it. Moreover, such a system has the advantage of being able to be embodied with a low-powered, and consequently less expensive, light source.
Unfortunately, the phenomena are rendered complicated by virtue of the tensorial nature of the Pockels effect. In addition, the tensor of the Pockels effect generally includes 18 positive rij coefficients (with 1.ltoreq.i.ltoreq.6 and 1.ltoreq.j.ltoreq.3) and the same phase variation may be obtained for several electric field orientations and values.
Electric field sensors using the Pockels effect in crystals with weak symmetry have already been embodied and are described in a publication by J. Chang and C. N. Vittitoe and entitled "An electro-optical technique for measuring high frequency free space electric fields", Fast Electrical and Optical Measurements vol. 1, Current and Voltage Measurements, Martinus Nijhoff publishers, Dordrecht 1986, published by J. E. Thompson and L. H. Luessen, pp. 57-71, and in a publication by K. D. Masterson and entitled "Photonic electric field probe for frequencies up to 2 GHz", SPIE Proceedings, vol. 720 (1986), pp. 100-104.
These sensors have the drawback of only making it possible to determine the value of electric fields for which the direction is firstly known.
However, this drawback does not appear in strong symmetry crystals for which a large number of coefficients of the Pockels tensor are identically nil. In particular, in the case of cubic crystals, there are no more than three coefficients, which are all equal. In this case of symmetry, I. P. Kaminow showed in 1974 in "An Introduction to Electrooptic Devices", Academic Press, New York, pp. 40-41, which propagation and polarization directions could be used to independently measure the electric field in three perpendicular directions.
Sensors based on this method for measuring the electric field via the Pockels effect in cubic crystals has been described in the document by B. N. Nelson and al and entitled "Fiber optic electric field sensor configurations for high bandwidth lightning research measurement applications" SPIE, vol. 720 (1986), pp. 85-90.
These sensors use optical fibers to guide the incident beam entering the crystal and to guide the beam emerging from this crystal. The polarization of the incident beam is embodied by polarizers placed immediately in front of the crystal and the beam leaving the crystal is effected with an analyzer placed immediately behind the crystal. These polarizers and analyzers are situated in the measuring probe, which has the drawback of increasing the dimensions of the latter and thus prevents the electric field from being measured in the required locations. Furthermore, the crystal functions on transmission, which prohibits the carrying out of measurements of the electric field close to a wall.
In addition, and this is more serious, no care has been taken as regards the macroscopic geometric symmetry of the probe crystal. Now, the electro-optic crystals used have a dielectric constant or more than 10 (16 in the case of bismuth germanate BI.sub.4 Ge.sub.3 O.sub.12). Also, such a crystal with any shape may induce distortions in the electric field lines, which ensures that such sensors provide a good order of magnitude of the measured electric field, but also mean that the real value and the direction of this field supplied by these sensors are in no way guaranteed. Accordingly, these sensors are not reliable.
Furthermore, the probe crystals used are cut into the shape of cubes and thus present an extremely high vibrational resistance as regards their response to their own vibration frequency of c/2a where c is the sonic speed in the crystal and a the side of the cube. To take the example of a bismuth germanate sensor with a side of 1 cm, this actual frequency is about 400 kHz and significantly adversely affects electric impact research.
The general and vague problem of "distortion" of the electric field to be measured has already been posed in the document by Tanaka JP-A-58 113 764. So as to resolve this problem, Tanaka recommends using as a sensitive Pockels effect element a parallelpiped-shaped component comprising, on two opposing sides perpendicular to the electric field to be measured, dielectric films whose dielectric constant has an intermediate value between that of the sensitive element and that the ambient environment allowing for a more progressive variation of the dielectric constant.
However, there are many causes of this "distortion" and the problem of "distortion" the invention seeks to resolve concerns the discrepancy between the directions of the electric field inside the crystal and the external field to be measured.